Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

Place the following acid chlorides in the decreasing order of reactivity to hydrolysis. (1) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl}\) (2) \(\mathrm{p}-\mathrm{O}_{2} \mathrm{NC}_{6} \mathrm{H}_{4} \mathrm{COCl}\) (3) \(\mathrm{p}-\mathrm{CH}_{3} \mathrm{OC}_{6}^{+} \mathrm{H}_{4} \mathrm{COCl}\) (a) \(2>1>3\) (b) \(3>1>2\) (c) \(1>2>3\) (d) \(1>3>2\)

Step-by-Step Solution

Answer

The order of reactivity is (a) 2 > 1 > 3.

Step 1: Understanding Hydrolysis Reactivity

The reactivity of acid chlorides towards hydrolysis depends on the electronic effects of substituents on the benzene ring. Electron-withdrawing groups increase reactivity, while electron-donating groups decrease reactivity.

Step 2: Analyzing Compound (1) - Benzoyl Chloride

Compound (1) is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl}\). The benzene ring has no substituents apart from hydrogen; therefore, this is the standard reactivity without electron-withdrawing or donating groups influencing it.

Step 3: Analyzing Compound (2) - Nitrobenzoyl Chloride

Compound (2) is \(\mathrm{p}-\mathrm{O}_{2} \mathrm{NC}_{6} \mathrm{H}_{4} \mathrm{COCl}\). The nitro group is a strong electron-withdrawing group through resonance and induction, which increases the reactivity of the acid chloride to hydrolysis.

Step 4: Analyzing Compound (3) - Methoxybenzoyl Chloride

Compound (3) is \(\mathrm{p}-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{4} \mathrm{COCl}\). The methoxy group is an electron-donating group through resonance, which decreases the reactivity of the acid chloride to hydrolysis.

Step 5: Ordering Based on Reactivity

Based on the electronic effects, compound (2) with the nitro group is the most reactive, followed by compound (1) without any substituents, and lastly, compound (3) with the methoxy group.

Key Concepts

Understanding Hydrolysis in Organic Chemistry

Hydrolysis is a fundamental reaction within organic chemistry that involves the breaking of a bond through the addition of water. Specifically, with acid chlorides, hydrolysis results in the conversion of the acid chloride into a carboxylic acid.
For students, it's essential to grasp that the process of hydrolysis is influenced by different factors like the nature of substituents attached to the molecule, which can either accelerate or decelerate the reaction.
The overall mechanic involves the nucleophilic attack of the water molecule on the electrophilic carbon in the acid chloride group. This is followed by a pairing of steps that lead to the displacement of the chlorine atom, forming the carboxylic acid.

The Role of Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a significant role in organic reactions, including hydrolysis. These groups pull the electron density away from the atom to which they are bonded.
In the context of acid chlorides, electron-withdrawing groups increase the reactivity towards hydrolysis. They make the carbon atom of the acid chloride more electrophilic, thus more susceptible to the nucleophilic attack by water.
One example from our exercise is the nitro group (NO₂), known for its strong electron-withdrawing ability through both resonance and inductive effects. This group enhances the reactivity of the acid chloride significantly, making the compound undergo hydrolysis more readily than others.

Impact of Electron-Donating Groups

On the flip side, electron-donating groups (EDGs) like the methoxy group can significantly reduce the reactivity of a compound towards hydrolysis. These groups push electron density towards the atom or group to which they are attached.
With acid chlorides, the electron-donating nature reduces the electrophilic character of the carbon atom, making it less likely to engage in nucleophilic attack from water.
In the given set, the methoxy group serves as an electron-donating group that decreases the reactivity of the acid chloride in our exercise. When compared, compounds with EDGs are less reactive than those with no substituents or those with EWGs.

Integrating Organic Chemistry Concepts

Organic chemistry involves the study of the structure, properties, reactions, and synthesis of carbon-containing compounds. In our discussion about hydrolysis and the effects of substituents, it's clear how chemical behavior can change dramatically with different functional groups.
Understanding these concepts provides a window into predicting and designing synthetic pathways for complex organic molecules.
Students should focus on how the presence of electron-withdrawing and electron-donating groups affect the reactivity of molecules. This knowledge can be applied broadly, not just in hydrolysis, but in other reactions where nucleophiles and electrophiles play a critical role.